Uniformity variations and hue shifts attributable to the combined effects of the imager and lens have been corrected in cameras manufactured by Eastman Kodak Company, Rochester, N.Y., as early as 1991. In these cameras, uniformity and hue correction are implemented as arrays of gain values to be applied to the image data. In this document, arrays are two-dimensional and are in some ways treated as an image. There is exact correspondence between rows and columns in an image and the rows and columns in an array. Similarly, a map is a two-dimensional array of values with exact correspondence between the rows and columns of the map and the rows and columns of the image. The map contains numerically encoded logical information about the pixels in the corresponding image. For example the value in a map might be set to zero if the pixel with the corresponding row and column is less than a first threshold, the value might be set to one if the pixel with the corresponding row and column is within a range of values, and it could be set to two if the pixel with the corresponding row and column is above a second threshold.
Electronic cameras (and scanners) have incorporated gain uniformity correction for many years. These gain corrections have usually been calibrated with a specific lens. In the case of a camera with interchangeable lenses, not all lenses have the exit pupil at the same location. As shown in FIG. 1, the exit pupil location can affect the uniformity of response for an electronic imager. Notice the ray angles get steeper at image locations further off axis. In the case of a monochrome imager, this could not affect the color of the response. In the case of an imager with a color filter array (CFA), adjacent pixels are typically different colors, such as shown in the Bayer pattern of FIG. 2. In the CFA pattern shown, pixels marked R are sensitive to red light, pixels marked B are sensitive to blue light, and pixels marked GoR or GoB are sensitive to green light. Because each pixel has neighbors of a different color, cross talk between pixels introduces hue shifts. FIG. 3 shows a closer view of light rays striking an imager with a CFA at varying incident angles. The pixel on the optical axis has very little potential for cross talk but the cross talk is much higher at the edge pixels because the light passes through the CFA for one color pixel and falls, at least partially, on a photo site for the next pixel. This effect increases from center to edge and leads to a hue shift from center to edge. The effect is lens, aperture, and zoom setting dependent because the ray angles change depending on exit pupil size and position. If the exit pupil is closer to the imager, the light rays approach from a wider range of angles; if the exit pupil is farther from the imager, the range of angles is narrower.
Digital cameras sold by the Eastman Kodak Company use a hue correction method where the gain required to correct each pixel to provide equal sensitivity and consistent hue is calculated and stored in an array. The values in the arrays tend to change slowly and smoothly. For efficiency, the arrays are paxelized and then the required value for each pixel is interpolated from the reduced size array. An image or array is paxelized by creating a smaller image or array that is the result of an average of every “N” values vertically and “M” values horizontally where “N” and “M” are integer numbers of pixels. Typically N and M are equal but they do not have to be equal.
The gain correction arrays in Kodak cameras are developed with a well-chosen lens, and are effective for all but a few pathological lenses. The electronic imagers in earlier Kodak cameras covered only a portion of the lens field so different lenses did not introduce, significantly different effects because the range of ray angles in the reduced field image is also reduced. With the introduction of an imager covering the full field of a 35 mm SLR lens (24 mm by 36 mm), the effects due to lens variation became significant so an additional adjustment of the correction became necessary for these cameras. Cameras with smaller imagers will also have hue shift that varies significantly if they have lenses with a large variation in exit pupil location, especially with exit pupils near the imager.
One instance of prior art in the patent literature is United States Published Patent Application US 2003/0234879 A1, “Method and Apparatus for Color Non-uniformity Correction in a Digital Camera,” C. A. Whitman et al, assigned to Hewlett Packard Company, published Dec. 25, 2003.
The method described by Whitman et al, stores multiple gain correction arrays. Each array accurately applies only for a specific lens condition (focal length and aperture). They teach a method of storing arrays for different conditions, and interpolating between arrays for intermediate conditions. This method requires detailed characterization of a specific lens.